Lignite is the youngest, most chemically reactive species of the family of coals found throughout the world. Typically low in ash and sulfur but high in volatile matter, United States lignites have moisture contents in the range of 30-40% and carbon levels of 40-50%. Lignites are surface mined, principally in North Dakota and Texas, with approximately 110 million tons mined in 1990. North Dakota has over 80% of the lignite in the continental U.S. with a total resource in excess of 500 billion tons.
Lignite char is produced by heating raw lignite at temperatures sufficient to drive off most of the water (&gt;400.degree. C.) and the other volatile materials (tar oils). In this process, known as "carbonization," the remaining char particles become highly porous as the volatile liquids are driven off. The resulting chars typically have carbon contents in the range of 59-82%; ash contents of 9-28%; and 5-12% volatile matter.
In the past, conventional thinking was that lignite chars, that is low rank chars, could not be used for making effective filters, primarily because of their high moisture content and their typically, as tested, lack of high adsorption capacity, particularly for materials developed in power station fuel-gas, such as sulfur dioxide, nitrogen oxides and heavy metal ions, such as mercury. Nevertheless, there has been some prior work with lignite in general as a possible char adsorbent. In particular, activated carbons and char adsorbents from "brown coal" and lignite have been produced for several decades. A material known as F-Coal was produced in the former German Democratic Republic and a similar S-Coal was produced in the USSR dating back to the 1950's. These were made from "brown coal", semi-coke by activation with a steam and CO.sub.2 mixture. Their principal application was for desulfurization applications. An activated carbon known as Darco, derived from Texas lignite, has been produced by the Atlas Chemical Company in the U.S. Presently, the largest known producer of lignite char/coke for adsorbent applications is Rheinbraun AG of Koln, Germany. The char is produced from the brown coal of the Rhine Valley region and sold principally as a flue-gas sorbent in a process developed jointly with Stadtwerke AG.
Technical work on lignite char adsorbents and activated carbons dates back to the early part of the century at the University of North Dakota. A 1937 report by C. R. Bloomquist, "The Production of Activated Carbon from Lignite," summarizes work by University of North Dakota researchers and others on early attempts to produce a good adsorbent from lignite feedstocks.
A 1963 study by Thelan investigated the feasibility of producing an activated carbon from North Dakota lignite in a fluidized bed reactor. Carbonized lignite was reacted with steam at elevated temperatures up to 950.degree. C. Amick and lavine and, in a separate study, Cooley had conducted earlier trials in producing an activated carbon for comparison against the Darco product being produced by the Atlas Chemical Company from Texas lignite. Their initial trials had yielded a product which gave a benzoic acid adsorption considerably higher than that of Darco.
Batch tests by Thelan indicated that steam activation became significant at temperatures over 540.degree. C. with increasing benefit up to 1040.degree. C. (the limit of his equipment). At the upper limit of activation, the activated char from North Dakota lignite had greater adsorption capacity than one of the Darco products, but less than a second product. The optimal conditions, based on limited testing, appeared to be produced by activation at 950.degree. C. with a particle size of -40, +60 mesh, a steam rate lower than 0.35 ft.sup.3 /min, and a char feed rate of 300 gm/hr into a fluid bed reactor.
In 1969, an unpublished thesis study by Schroeder focused on "The Production of Activated Carbon from North Dakota Leonardite." The goal of the study was to assess the adsorptivity potential of an activated carbon produced from a North Dakota Leonardite. Since Leonardite has lost much of its structural integrity, relative to lignite, the study focused on producing a granular activated carbon using a method of colloidal suspension and precipitation followed by drying to yield hard dense granules. A method of pelletizing powdered Leonardite by spraying a 5% solution of sodium hydroxide onto the powdered Leonardite in a rotating drum was investigated.
The Schroeder study produced a Leonardite activated carbon with a bulk density of 0.5 gm/ml in a 8-14 mesh size. Carbonization was carried out at 294.degree. C. followed by cooling by a water mist and then activation in a steam environment at 925.degree. C. The maximum adsorptive power was displayed from a pelletized Leonardite with a zinc chloride additive yielding a higher adsorptive level than Darco S-51 (Texas lignite based) and 60% of Columbia Grade C (coconut shell based).
The study recommended using zinc chloride and calcium hydroxide before briquetting to increase the adsorptive properties. Likewise a precarbonization step before briquetting may enhance granule strength.
In 1970, McNally prepared another thesis entitled, "Activated Carbon from North Dakota Lignite," designed to determine the prevailing factors in the activation process. A lignite-based activated carbon was produced which demonstrated an iodine adsorption capacity of 0.725 gm/gm C compared to 0.55 gms/gm C for a commercial Texas lignite char being produced at the time. The study concluded that the effects of activation residence time ranging between 6-24 minutes has a greater impact on adsorption capacity than activation temperatures in the range of 775.degree.-950.degree. C.
The study recommended the use of a fluidized bed reactor for activation and the reduction of ash from the char by leaching. The study also suggested that future studies should investigate the direct activation of raw and dried lignite and that lignites from different mines be characterized for adsorption capacity.
With the general advent of desulfurization systems for industrial and utility emissions control in the 1970's and 1980's, more attention was given to finding low-cost feedstocks for producing adsorbent carbons. Kassebohm, et al. 14th Annual Lignite Symposium, pp 2 A1:1-16, 1987, described the work of Stadtwerke AG in using brown coal chars in fluegas adsorbers in the Rhine Valley region of the Federal Republic of Germany. G. Q. Lu and D. D. Do, Carbon Vol. 29, NO.2, pp 207-213, 1991, also report preparation of a sorbent for SO.sub.2 and NO.sub.x removal using coal washery reject in Australia. This solid waste material generated in coal preparation processes was pyrolyzed at around 550.degree. C. and then activated with carbon dioxide at 900.degree. C.
Lu and Do reported using carbonaceous sorbents prepared from these wastes to remove SO.sub.2 at 100.degree.-200.degree. C. resulting in sulfuric acid production. After the SO.sub.2 removal stage, the flue-gas was cooled down to around 25.degree.-50.degree. C. to achieve primarily NO.sub.2 and subsequent adsorption on the char. The NO.sub.2 was then desorbed at 200.degree.-350.degree. C. to generate a gas stream of concentrated carbon dioxide, nitrogen, NO, and CO. The NO is then catalytically reduced in the presence of CO. The equations are as follows: EQU [C-NO.sub.2 ]=NO+CO+CO.sub.2 +N.sub.2 EQU 2NO+2CO=N.sub.2 +2CO.sub.2
This process replaces the use of expensive sorbents like mixed metal oxides and activated carbons without using ammonia at the expense of using some carbon in the NO.sub.x reduction mechanism.
Tests conducted in producing chars from this waste material showed an optimal carbonization temperature of 565.degree. C. at four hours and followed by activation with CO.sub.2 at 900.degree. C. NO.sub.x adsorption was increased by nearly a factor of two with activation, whereas SO.sub.2 adsorption for this material was not significantly affected. The study by Lu and Do demonstrated the importance of pyrolysis and activation temperatures in developing the optimum porous infrastructure prior to activation. The washery waste may become an economically viable flue-gas adsorber material.
In summary, the literature highlights the need to identify the optimal carbonization temperature which yields the maximum porosity infrastructure prior to any activation process. This temperature will be specific to the carbon feedstock but usually lies in the range of 400.degree.-600.degree. C. Activation by steam or carbon dioxide is usually optimized in the range of 850.degree.-950.degree. C. Slower carbonization processes at lower temperatures tend to yield a more porous infrastructure by driving off nearly all the volatiles without consuming the carbon. The optimal activation temperature will gasify surface carbon to the extent that the pores produced in carbonization are increased in size without consolidating them resulting in a loss of internal surface area.
Nearly all studies to date indicate enhanced adsorption of particularly SO.sub.2 and NO.sub.x in the presence of basic surface oxygen radicals. The presence of alkaline ash, while not as decisive, also tends to be positive by further disrupting the graphitic-like surfaces of carbon thereby creating additional basic adsorption points.
It can be seen from the above described state of the art, that while there have been some processes developed for producing chars from lignite, none have been successful in producing a lignite char that has the full range of capability of far more expensive activated carbon, particularly from the standpoint of developing a filter char which can be used particularly as a carbonaceous sorbent for sulfur dioxide and nitrogen oxide removals, particularly in power plant flue-gas streams.
Accordingly, it is a primary objective of the present invention to produce a carbon adsorbent material from low rank coal with an optimized adsorption capacity for oxides of sulfur and nitrogen and also heavy metals (i.e. mercury, cadmium, lead, etc.). It also adsorbs volatile organic compounds including the elements which make up dioxins and furans.
Another objective of the present invention is to provide carbon adsorbents of the above described undesirable pollutants that have the capacity to nearly completely remove these chemical substances from flue-gas and industrial process streams to achieve full compliance with environmental regulations.
Another objective of the present invention is to provide an inexpensive carbonaceous absorbent material which can fully compete with more expensive activated carbons in price, and at the same time, provide at least as efficient filtration, and in many instances, more efficient filtration than the more expensive activated carbons.
The method and manner of achieving the above objectives will become apparent from the description of the invention given hereinafter. It will be apparent to those of ordinary skill in the art that certain modifications can be made in the process and conditions hereinafter described and yet still achieve the objectives of the present invention.